Shutter Speed and Aperture

The way shutter speed and aperture work together is very important as it determines
whether or not the exposure for the image is correct.

The shutter is like a curtain within the body of the camera which opens and closes
briefly when you press the button allowing light to enter. It is the shutter speed which
controls just how much light is allowed to get in. With faster the shutter speed, the shorter
the amount of time and the less light is let in. So for example, a shutter speed of 1/4th
of a second is shorter than a shutter speed of 1/2 of a second so 1/4 lets less light
in as it is open for a shorter period of time.

Illustration of a camera diaphragm in three positions (Illustration by Lozère)

The aperture controls how much light enters the lens and varies the way the pupil of the
eye varies. Large apertures let in more light than small apertures (see digram above).
The opening of the aperture is controlled by the f-stop or aperture setting on
your camera. An aperture of f/16 is smaller than an aperture of f/5.6 and so it lets
less light in. Bigger f-stops let less light in and will give a darker picture; smaller
f-stops let more light in and will give a lighter picture.

going from f4 to f2.8 is 1 stop brighter

going from f16 to f8 is 2 stops brighter

going from f5.6 to f8 is 1 stop darker

Balancing Shutter Speed and Aperture

To get the exposure correct, you need to balance the shutter speed with the aperture as
they work together. With smaller apertures less light enters so the camera needs more
time to compensate, in other words a slower shutter speed.

The importance of this is that with small apertures (such as f/16) you get greater depth-of-field
(see image left, f/7.1) which means more of your scene will be in focus. Good for landscapes! With large
apertures (such as f2.8) you get a smaller depth-of-field which means less of your scene will be in
focus - good for portraits where the background needs to be blurred or macro shots
such as flowers and insects where you only want the foreground to be sharp (see image below, f/3.5).

So set your aperture according to the subject and then choose the shutter speed accordingly. If
you move up two stops in aperture, you can step down two stops in shutter speed. If you move
down one stop in aperture, you can step up one stop in shutter speed and so on. This way
the exposure level will remain constant but you can be creative about the shot.

Controlling exposure in this way will become automatic with practice but if you would
like to expand your knowledge of this subject, Understanding Exposure by
Bryan Peterson is an excellent next step. Peterson explains in greater depth the relationship between aperture and shutter speed, including
how to achieve successful exposures in seemingly difficult situations. Here is what
is covered:

which aperture to give you the greatest contrast and sharpness, and when to use it

which apertures guarantees the background remains an out-of-focus tone

which one aperture - when combined with the right lens - creates an area of sharpness from three feet to infinity

how to creatively use shutter speed to either freeze an action or imply motion

where to take a meter reading when shooting a sunset, snow, or a city at dusk